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WaterOperator.org Blog

Articles in support of small community water and wastewater operators.

Managing Dissolved Oxygen in Activated Sludge Plants

Managing Dissolved Oxygen in Activated Sludge Plants

Sustaining optimal dissolved oxygen levels in activated sludge plants is necessary for biological treatment of organic material and ammonia. While raw wastewater often contains some amounts of oxygen, aeration systems can increase dissolved oxygen (DO), mixing, and the suspension of microbes through mechanical agitation or diffused aeration. Aerobic microorganisms use this oxygen to breakdown organic waste into inorganic byproducts. The amount of dissolved oxygen consumed by microbes during biological treatment is referred to as biochemical oxygen demand (BOD). According to an article by Triplepoint Water, approximately 1.5 pounds of oxygen is consumed for every pound of BOD oxidized. To oxidize one pound of ammonia, that value increases to 4.57 pounds of oxygen. Most plants aim to maintain around 2 mg/L of DO which allows microbes contained within the center of floc to receive oxygen.

Wastewater operators should regularly monitor oxygen availability in the form of dissolved oxygen. Insufficient oxygen levels will allow aerobic and nitrifying microbes to die and floc to break up. At DO concentrations under 1 mg/L, the potential for filamentous growth increases. On the other end of the spectrum, too much oxygen increases power consumption and, at very high levels, inhibits settling. Research has estimated that aeration can use up to 45 to 75% of a treatment facility’s overall electricity use. With an online DO analyzer equipped to automated controls, the EPA reports that energy costs can be reduced by as much as 50%.

Where and when an operator samples for DO will be determined by the requirements written in the facility’s National Pollutant Discharge Elimination System (NPDES) permit and basic process control. To compare dissolved oxygen levels throughout the day, samples should be collected at the same location. The Ohio EPA’s Activated Sludge Process Control and Troubleshooting Chart Methodology recommends that systems sample within 1-2 feet of the surface water near the discharge of the aeration tank into the clarifier. By collecting multiple samples in the same location throughout the week, operators can reliably determine if DO concentrations are sufficient for treatment while developing a DO profile. In addition, measuring DO at multiple depths and locations in the aeration tank can help find dead spots.  

To supply adequate DO, the Ohio EPA manual includes how to determine blower runtime based on organic loading and system design. We should  still note that temperature, pressure, and salinity can all influence the solubility of oxygen. Additional sampling locations can include the raw wastewater, aerobic/ anaerobic digester, and final effluent. Final effluent with high dissolved oxygen can cause eutrophication in the receiving waters, however low DO can harm aquatic organisms. Some permits set a minimum DO level for effluent to ensure aquatic organisms have the necessary oxygen levels to sustain life.

While every technique and tool has its strengths and weaknesses, operators can measure DO through a Winkler Titration test (see Michigan DEQ Laboratory Training Manual pg.91), electrochemical sensor, or optic sensor. The two sensors mentioned can be purchased as portable handheld meters or stationary devices. For automated blower control and continuous sampling, an online sensor is used. For NPDES compliance monitoring, measurements must be taken through an EPA approved method at the frequency specified in the permit.

When using any DO sensor, the EPA’s Field Measurement of Dissolved Oxygen (SESDPROC-106) procedures require that the equipment be well maintained and operated per manufacturer instructions. Upon initial purchase, probes should be inspected, calibrated, and verified for accuracy. During each additional use the instrument should be calibrated and inspected again. The EPA recommends checking instrument calibration and linearity using at least three dissolved oxygen standards annually. All maintenance and sampling activities should be documented in a logbook per NPDES requirements. Any time a measurement is taken, the temperature of the water and any notable wastewater conditions should also be recorded in the logbook. 

Dissolved oxygen is a frequently monitored parameter in wastewater treatment systems. Operators should have a firm understanding of how dissolved oxygen is involved in wastewater processes and how they can manage DO to achieve compliance. Check out our online document library to find useful resources to learn more.

Featured Video: How to Use a Hydrant Sampler

Featured Video: How to Use a Hydrant Sampler

Through the use of a hydrant sampler, operators can monitor water quality at various points in the distribution system without the need for access to indoor taps from local businesses or residential homes. Sampling hydrants allows operators to protect public health by routinely collecting bacteriological samples required by their regulatory agency. Operators should sample along the distribution system at the locations and frequency specified by their RTCR sample siting plan. For assistance in developing or updating your sampling plan, check out the EPA documents Sample Siting Plan Instructions (download) and the Revised Total Coliform Rule (RTCR) Sample Siting Plan with Template. Please check with your Primacy Agency to determine if stricter requirements may apply to your system.

In this week’s featured video, the U.S. EPA’s Area-Wide Optimization Program demonstrates how to use a hand-built hydrant sampler on dry barrel hydrants to collect water quality samples throughout the distribution system. The procedures used  in this video, including how to calculate flush time and how to build a sampler, can be found at the EPA’s Hydrant Sampler Procedure and Parts List web page. Calculating an appropriate flush time is important to yield sample results that accurately characterize the quality within your distribution system. The hydrant sampler from the video can be built with parts from your local hardware store however, since 2018 AWOP has created a new sampler design that requires less parts making it cheaper to build and easier to use. Check out this week’s featured video to find out the best practices and safety concerns for using a hydrant sampler.

Featured Videos: Invisible Heroes, Minnesota's Drinking Water Providers

This week's featured videos are part of a new series produced by the Minnesota Department of Health showcasing the "invisible heroes" of Minnesota's drinking water supply. In these 3-minute videos, small town water system heroes face and overcome a variety of challenges including contamination, source water shortages and aging infrastructure in order to provide safe, reliable water for their communities. Three of the videos feature small or very small water systems and the innovative strategies and partnerships they have developed to overcome their challenges. 

The first video looks at how the tiny community of St. Martin (pop. 350) has become the first town in the state with a biologically active treatment plant in order to effectively respond to high levels of iron and ammonia in their water. 


The next video explains the unique wellhead protection program developed by the City of Worthington, MN (pop. 13,000). In order to protect the City's drinking water wells from contamination, the city, along with partner Pheasants Forever, created the Worthington Wells Wildlife Management Area. 


And finally, here is a video about how the small city of Fairmont, MN (pop. 10,000) sprang into action when faced with increasing nitrate levels. 


What do all three of these smaller systems have in common? They worked collaboratively with the Minnesota Department of Health to ensure their strategies would meet with success! 

EPA’s Arsenic Rule Results in Fewer Incidents of Cancer

EPA’s Arsenic Rule Results in Fewer Incidents of Cancer
A recent New York Times article reports that the EPA’s revised rule on arsenic contamination in public drinking water systems has resulted in fewer lung, bladder and skin cancers. This finding, published last month in Lancet Public Health journal, is the result of a study that compared the urinary arsenic levels of over 14,000 people in 2003, before the new rule went into effect, to those in 2014, well after the rule had been fully implemented. The researchers found a 17 percent reduction in arsenic levels in this time period and they estimate that this reduction has resulted in 200-900 fewer lung and bladder cancers and 50 few skin cancers annually.

This finding is reassuring to water systems that have spent time, money and effort on arsenic rule compliance – it is always good to know that regulations are truly making a difference in the lives of community members.

It also highlights the importance of water systems, and especially those with groundwater sources, working with their local and state officials to determine the best way to test for arsenic and, if necessary, treat their water supply.  And because two water systems with similar levels of arsenic in their source water often need two entirely different types of treatment technology, and because these technologies can be expensive, knowledge about arsenic compliance, treatment and funding sources is essential.

Luckily, WaterOperator.org can help point you in the right direction when you choose "arsenic" as the category in our document database. A good first stop is also this EPA webpage which offers lots of resources and tools to operators, such as a rule summary and steps to take towards compliance.

Top 2017 Resources from WaterOperator.org's Bi-Weekly Newsletter

Top 2017 Resources from WaterOperator.org's Bi-Weekly Newsletter

2017 was a great year for the WaterOperator.org newsletter team. We not only reached our 200th edition milestone this past fall, but we also were successful in connecting a significant number of water professionals with useful and relevant resources, resources that could be used on-the-spot to solve pressing issues, or help guide utility best practices, or help water decision-makers plan ahead for their communities. 

While many of the events, articles and resources featured in our newsletters garnered interest, here is a list of our most clicked-on resources of 2017.

Did you use one these resources at your utility this year? If so, we'd love to hear from you! Do you have a favorite "go-to" resource to share? Again, we'd love to know! Our email is info@wateropertor.org , or connect with us on Facebook or Twitter